The three main functional components of a laser

The three main functional components of a laser are the pump source, the gain medium, and the resonant cavity.

The pump source provides the light source for the laser. The gain medium (also known as the working medium) absorbs the energy provided by the pump source and amplifies the light. The resonant cavity forms the circuit between the pump source and the gain medium, and the resonant cavity oscillates in a selected mode to produce laser light.

The pump source, as an energy source, generates photons to excite the gain medium. The photons emitted by the pump source pump the particles in the gain medium from the ground state to a higher energy level, achieving population inversion. Excitation mechanisms include optical excitation (optical pumping), gas discharge excitation, chemical excitation, and nuclear excitation. Currently, high-power semiconductor lasers (LDs) are commonly used as pump sources, primarily for converting electrical energy into light energy.

The gain medium achieves population inversion and amplifies light, and also determines the wavelength of the output laser. Gain media can be liquids, gases, or solids. Liquids include organic solutions, gases include carbon dioxide, and solids include ruby. The fundamental requirement for a gain medium is that it generates photons upon stimulation rather than converting light into heat. The particles within it must be relatively isolated, allowing transitions between energy levels to occur.

A resonant cavity primarily serves the purpose of "storing" and "purifying" laser light. A resonant cavity typically consists of two mirrors, but couplers can also be used to form various ring resonators. Photons bounce back and forth between the mirrors, continuously inducing stimulated radiation in the gain medium and generating high-intensity laser light. Furthermore, the resonant cavity ensures that the photons within the cavity have consistent frequency/wavelength, phase, and direction, resulting in excellent directivity and coherence of the laser light.